If such qualities of style and presentation were encouraged by the environment in which Aquinas pursued his earlier studies, they were also helpful in the task which he chose as his life-work.  This was the construction of a system in which all the elements of knowledge should be harmoniously united.  An undertaking so vast necessitated a long preparation, the study of all available sources, and the elucidation of many detailed problems.  Hence, a considerable portion of St. Thomas’s works is taken up with the explanation of Peter Lombard’s ‘Sententiae,’ with Commentaries on Aristotle, with Expositions of Sacred Scripture, collections from the Fathers, and various opuscula or studies on special subjects.  Under the title ‘Quaestiones Disputatae,’ numerous problems in philosophy and theology are discussed at length.  But the synthetic power of Aquinas is shown chiefly in the ‘Contra Gentes’ and the ‘Summa Theologica,’ the former being a defense of Christian belief with special reference to Arabian philosophy, and the latter a masterly compendium of rational and revealed truth.

The conception of the ‘Summa’ was not altogether original.  From the earliest days of the Church, men of genius had insisted on the reasonableness of Christian belief by showing that, though supernatural in its origin, it did not conflict with either the facts or the laws of human knowledge.  And as these had found their highest expression in Greek philosophy, it was natural that this philosophy should serve as a basis for the elucidation of revealed truth.  The early Fathers turned to Plato, not only because his teaching was so spiritual, but also because it could be so readily used as a framework for those theological concepts which Christianity had brought into the world.  Thus adopted by men who were recognized authorities in the Church,—­especially men like Augustine and the Areopagite,—­Platonism endured for centuries as the rational element in dogmatic exposition.

Scholasticism inaugurated a new era.  Patristic erudition had gathered a wealth of theological knowledge which the Schoolmen fully appreciated.  But the same truths were to receive another setting and be treated by different methods.  Speculation changed its direction, Aristotle taking the place of his master.  The peripatetic system found able exponents in the earlier Scholastics; but Aquinas surpassed them alike in the mastery of the philosopher’s principles and in his application of these principles to Christian doctrine.  His Commentaries on Aristotle adhere strictly to the text, dissecting its meaning and throwing into relief the orderly sequence of ideas.  In his other works, he develops the germs of thought which he had gathered from the Stagirite, and makes them the groundwork of his philosophical and theological speculations.